Gravity and Magnetics

3-D focusing inversion of the ground and airborne gravity, magnetic, and gravity gradiometer data

*By Michael S. Zhdanov,
Robert Ellis, and Souvik Mukherjee*

CEMI has applied the regularized focusing inversion to 3-D gravity, 3-D magnetic, and 3D gravity gradiometer data inversion. Our inversion method is designed for inversion of the anomalous gravity and/or any component of the anomalous magnetic field, including a total magnetic anomaly observed in the ground or airborne surveys (Portniaguine and Zhdanov, 2002; Zhdanov, 2002; 3-D gravity and/or magnetic inversion with image focusing and data compression

Recently we have developed also a method and a computer code for 3-D tensor gravity field inversion, based on ideas of data compression and image focusing (Zhdanov et al., 2003). The focusing inversion makes its possible to reconstruct a much more focused and much clearer image of the geological target than the conventional maximum smoothness inversion (Modeling and inversion of 3-D gravity tensor field). It seems that this new technique is suited quite well for the interpretation of gravity gradiometer data, which are sensitive to local density anomalies.

A new version of the GRMAG3D code for three-dimensional inversion of magnetic, gravity data, and gravity gradient data with image focusing was released recently to the sponsors. The current version (version 3) can process the gravity gradient data with an option for joint inversion of the different gravity tensor components. The program is written in the Matlab language, but it can also be run without Matlab. The Windows stand-alone executable of the program is provided along with the Matlab source codes. There is also a user friendly interface available to operate the code and to export and import data from the CEMI code to the Geosoft data base.

As an example we will present below the results of the application of this method for interpretation of the gradient gravity data collected by BHP Billiton over the Cannington Ag-Pb-Zn ore body in Queensland, Australia (Zhdanov et al., 2003) using BHP Billiton's FALCON(TM) airborne gravity gradiometer (AGG). AGG instrument is based on a design of opposing accelerometers with tangential sensing axes mounted on a slowly rotating wheel. The gravity tensor components gxy and gD = ½(gxx-gyy) are recorded. Figure 1 (top panel) shows the observed differential curvature component gD. The same data predicted for the model obtained by regularized focusing inversion are shown in the bottom panel of Figure 1. We have used for inversion a rectangular grid consisting of about 100,000 cells describing the anomalous density distribution. Figure 2 shows a comparison with the drilling results along one of the vertical cross-section of the inverse model. It demonstrates a remarkable correlation between the density anomaly reconstructed by the gravity gradient data and the true structure of the ore body. This result indicates that the emerging new geophysical technology of the airborne gravity tensor observations can improve significantly the practical effectiveness of the gravity method in mineral exploration.

REFERENCES

Portniaguine O., and M. S. Zhdanov, 2002, 3-D
magnetic inversion with data compression and image focusing:
Geophysics, **67**, No. 5, 1532-1541.

Zhdanov, M. S. 2002, Geophysical inverse theory and regularization problems: Elsevier, Amsterdam - New York - Tokyo, 628 pp.

Zhdanov, M. S., Ellis., R., and S. Mukherjee, 2003, Regularized focusing inversion of 3-D gravity tensor data: Proceedings of 2003 CEMI Annual Meeting, 57-84.

Zhdanov, M. S., Ellis., R., and S. Mukherjee, 2003, 3-D regularized focusing inversion of gravity gradient tensor data: Geophysics, submitted.